1. Field of The Invention
The present invention relates to a ferroelectric liquid crystal display device.
2. Description of the Related Art
Ferroelectric liquid crystals including a chiral-smectic C phase liquid crystal have an excellent characteristics such as a memory effect, a high response rate and a wide angle of view. Research has been actively conducted on an application of ferroelectric liquid crystals to display device with the most fine resolution, and the largest capacity.
At the outset, the principle of the behavior of ferroelectric liquid crystals will be detailed hereinbelow. FIG. 2(a) is a schematic representation illustrating a transfer path of a ferroelectric liquid crystal molecule FIG. 2(b) is a projection view of FIG. 2(a) as viewed from the direction shown by an arrow 8. Reference Numeral 10 designates a ferroelectric liquid crystal molecule.
The ferroelectric liquid crystal molecule is oriented parallel to substrates and has a layer structure formed in a direction perpendicular to the substrates. The ferroelectric liquid crystal molecule 10 is arranged in such a manner that it is inclined at a tilt angle .THETA. in a direction 13 or 15 relative to a normal line 9 of this layer. The ferroelectric liquid crystal molecule 10 exhibits a spontaneous polarization Ps with the result that the application of an electric field from the outside of the liquid crystal causes the liquid crystal to receive the force proportional to the vector product of this electric field and the spontaneous polarization Ps in a direction perpendicular to the longitudinal axis of the molecule to travel on a surface of the conical transfer path 12 with an apex angle twice as large as the tilt angle 2.THETA.. The driving force of the travel is given by the spontaneous polarization so that these kinds of ferroelectric liquid crystals exhibit a high response rate; only 1/100 or 1/1000 of that of the conventional liquid crystal using a nematic liquid crystal.
The ferroelectric liquid crystal molecule 10,14,16 is stable in two different states. It exhibits a stable state when a positive electric field E moves the molecule 10 to the axis 13 shown in FIG. 2(b) whereas it exhibits another stable state when a negative electric field moves it to the axis 15. These stable states are held unless no electric field required for the transfer of molecules are applied. In other words, the ferroelectric liquid crystal molecule has a memory properties.
Rendering either of the two stable states 14, 16, for example, the molecular axis of the stable state 14 identical to the absorption axis of the polarizing plate provides a black state impervious to light and whereas rendering the molecular is of the other stable state 16 identical to the absorption axis thereof provides a state of white color.
Operating the stable state of the molecule 10 in each pixel by matrix driving to offer a predetermined display state.
However, matrix driving results in applying a bias voltage to all the pixels, thereby producing a molecular motion to deteriorate the quality of the contrast.
In this connection, C1 uniform which generates only a small amount of light leakage because of the application the bias voltage and which reduces the deterioration of the contrast is employed as a orientational sate of the ferroelectric liquid crystal.
As other kinds of orientational sates, C1 twist and C2 uniform are known (see Ferroelectrics, 114, pp 3 (1991)), Liquid Crystal Discussion Drafts (1991, Koden), Japanese Laid-Open Patent Application No. HEI 2-165120 and the like). Both C1 twist and C2 uniform are low in contrast.
On the other hand, there has been proposed a driving method that can reduce the force unassociated with the rewriting of image applied on the liquid crystal molecule by employing a relationship between the applied voltage and the response rate seen in a ferroelectric liquid crystal having a dielectric anisotropy of less than 0 wherein a specific voltage applied minimizes the response rate while voltages on both sides of the specific voltage increases the response rate (see Japanese Laid-Open Patent Application No. SHO 62-56933/1987 and Japanese Laid-Open Patent Application No. HEI 1-24234/1989). The method will be detailed hereinafter.
A force applied to the ferroelectric liquid crystal stand proportional to the force generated by the spontaneous polarization Ps, a difference .DELTA..epsilon. in dielectric potential between the longitudinal axis and the transverse axis and the second power of the electric field E. In other words, the force E that works in the direction of the transverse axis is represented by the following equation (1): EQU F=K.sub.0 .times.Ps.times.E+K.sub.1 .times.E.sup.2 .times..DELTA..epsilon.(1)
(where K.sub.0 and K.sub.1 represent proportional constants)
The force F is in inverse proportion to the response rate.
FIG. 4 shows a relationship between the electric field E and the response rate of the liquid crystal with .DELTA..epsilon.&lt;0 as disclosed in Japanese Laid-Open Patent No. HEI 1-24234/1989. As shown in FIG. 4, the response rate is minimized in the vicinity of 30 V. Since the effect of K.sub.0 .times.Ps.times.E item is sufficiently larger than the effect of K.sub.1 .times.E.sup.2 .times..DELTA..epsilon. in a region where the anisotropy working on the ferroelectric liquid crystal having .DELTA..epsilon. less than 0 exhibits a small effect in the range of up to less than 30 V, the force F increases and the response rate decreases while the voltage increases. On the other hand, since K.sub.1 .times.E.sup.2 .times..DELTA..epsilon. item becomes large in a region where the anisotropy working on the ferroelectric liquid crystal exhibits a large effect at voltage larger than 30 V, the force F decreases and the response rate increases along with the increase in the voltage.
Japanese Laid-Open Patent Application No. HEI 1-24234 proposes a driving method using the above relationship as follows. FIG. 5 is a graph illustrating a waveform of the driving voltage in accordance with the present invention. In the graph, Reference Numerals (1) and (2) designate a voltage waveform applied to the scanning electrode L, (1) representing a select voltage waveform, (2) representing a nonselect voltage waveform. Numerals (3) and (4) in FIG. 5 designate a voltage waveform applied to the signal electrode S, (3) representing black rewriting voltage waveform (4) representing white rewriting voltage waveform. Numerals (5) through (8) in FIG. 5 designates a voltage waveform applied to a pixel when (1), (2), (3) and (4) are combined.
Rewriting a white state of the liquid crystal constituting a pixel A.sub.ij at the crossing points of the electrodes as shown in FIG. 3 into a black state involves the application of a voltage waveform shown by (3) in FIG. 5 to a signal electrode S.sub.j upon applying a voltage waveform shown by (1) in FIG. 5 to a scanning electrode L.sub.i, and the application of a voltage waveform shown by (5) in FIG. 5 to the pixel A.sub.ij. Rewriting the black state of the liquid crystal constituting a pixel A.sub.ij into the white state involves the application of the voltage waveform shown by (4) in FIG. 5 to the signal electrode S.sub.j upon applying the voltage waveform shown by (1) in FIG. 5 and the application of a voltage waveform shown by (6) in FIG. 5 to the pixel A.sub.ij. With respect to the other pixel A.sub.kj, a voltage waveform shown by (2) in FIG. 5 is applied to a scanning electrode L.sub.k whereas, a voltage waveform shown by (3) or (4) in FIG. 5 to a signal electrode S.sub.1 as a result. A voltage waveform shown by (7) or (8) in FIG. 5 is applied to these pixels so as not to change in the display of the pixel.
What is important about this method for driving the liquid crystal is that the absolute value of voltages -V.sub.a, V.sub.a shown by (5) or (6) in FIG. 5 is given as a voltage in the vicinity of 30 V at which the response rate exhibits the minimum value shown in FIG. 4 whereas the absolute value of -V.sub.a -2 V.sub.b, V.sub.a +2 V.sub.b is given as a voltage sufficiently larger than 30 V. With the condition of dielectric anisotropy of .DELTA..epsilon.&lt;0 given, the force working on liquid crystal molecules with the former voltage becomes larger than the force working on liquid crystal molecules with the latter voltage, the boundary between the two voltages being placed around 30 V with the result that display is not rewritten at the latter voltage. Besides, with an increase in the latter voltage unassociated with the rewriting of images, the force working on the liquid crystal molecule in the direction of the transverse axis reduces to inhibit the molecular motion of the liquid crystal, thereby actualizing a high contrast.
"The JOERS/ALVEY Ferroelectric Multiplexing Scheme" reported by P. W. H. Surguy et al. at FLC '91 describes this method. FIG. 8 shows a driving voltage waveform in another method. The driving method involves rewriting one screen in two fields, applying a driving voltage waveform shown in FIG. 8(a)in the first field and applying a driving voltage waveform shown in FIG. 8(b) in the second field. In this method, Numeral (4) in FIG. 8 designates a voltage waveform applied to the signal electrode S, indicating a holding voltage waveform unassociated with the rewriting process. Other Numerals (1) through (8) in FIG. 8 designates the voltage waveform same as shown in FIG. 5.
Rewriting pixel A.sub.ij from the white state into the black state involves the application of a rewriting voltage for generating the black state as shown by (3) in FIG. 8(a) to a signal electrode S.sub.j upon applying a select voltage shown by (1) in FIG. 8(a) to a scanning electrode L.sub.i in the first field and applying the voltage waveform shown by (5) in FIG. 8(a) to the liquid crystal molecules constituting the pixel, thereby rewriting the pixel into the black state. In the second field, a select voltage shown by (1) in FIG. 8 (b) to a scanning electrode L.sub.i while applying the holding voltage shown by (4) in FIG. 8(b) followed by applying the voltage waveform shown by (6) in FIG. 8(b) to the liquid crystal molecules constituting pixel A.sub.ij to hold the black state. The pixel is rewritten to the white state.
Rewriting pixel A.sub.ij from the black state into the white state involves applying the holding voltage shown by (4) in FIG. 8(a) to a signal electrode S.sub.j upon applying a select voltage shown by (1) in FIG. 8(a) to a scanning electrode L.sub.i in the first field to apply the voltage waveform shown by (6) in FIG. 8(a) to the liquid crystal molecules constituting pixel A.sub.ij holding the black state at the outset. In the second field, the voltage waveform shown by (5) in FIG. 8(b) is applied to a signal electrode S.sub.j upon applying a select voltage shown by (1) in FIG. 8(b) to apply the rewriting voltage shown by (3) in FIG. 8(b) for generating the white state to a signal electrode S.sub.j, thereby applying the voltage waveform shown by (5) in FIG. 8(b) to the liquid crystal molecules constituting pixel. A.sub.ij to rewrite the pixel into the white state.
With respect to the other pixel A.sub.ij (k.noteq.i), in the first field, a non-select voltage shown by (2) in FIG. 8(a) is applied to a scanning electrode L.sub.i whereas the voltage waveform shown by (3) or (4) in FIG. 8(a) is applied to a signal electrode S.sub.j, thereby applying the voltage waveform shown by (7) or (8) in FIG. 8(a) to the liquid crystal molecules constituting pixel A.sub.ij. In the second field upon applying the non-select voltage waveform shown by (2) in FIG. 8(b) to the voltage waveform shown by (3) or (4) in FIG. 8(b) is applied to the signal electrode S.sub.j to apply the voltage waveform shown by (7) or (8) in FIG. 8(b) to the liquid crystal molecules constituting pixel A.sub.ij. At this point, application of either of the above voltage does not cause a change in the display of pixel A.sub.ij. What is important about this method for driving the liquid crystal is that the absolute value of voltages -V.sub.s +V.sub.d, or V.sub. s -V.sub.d shown by (5) in FIG. 8(a) or (5) in FIG. 8 is given as a voltage in the vicinity of 40 V at which the response rate exhibits the minimum voltage shown in FIG. 6(a) whereas the absolute value of -V.sub.s -V.sub.d m, V.sub.s +V.sub.d (FIG. 8(a)(6), FIG.(b)(6)) is given as a voltage sufficiently larger than 40 V. With a condition of dielectric anisotropy of .DELTA..epsilon.&lt;0 given, the force working on liquid crystal molecules with the former voltage becomes larger than the force working on liquid crystal molecules with the latter voltage, the boundary between the two voltages being placed around 40 V with the result that the display is not rewritten at the latter voltage. Besides, with an increase in the latter voltage unassociated with the rewriting of images, the force working on liquid crystal molecules in the direction of the transverse axis reduces to inhibit the molecular motion of the liquid crystal, thereby actualizing a high contrast.
In addition, prior to the application of the rewriting voltage -V.sub.s +V.sub.d or V.sub.s -V.sub.d,-V+V.sub.d and -V.sub.d or V.sub.s -V.sub.d and V.sub.d having same polarity, either voltage -V.sub.d or V.sub.d both are applied to reduce the rewriting voltage -V.sub.s +V.sub.d or V.sub.s -V.sub.d as liquid crystal molecules are placed in a state of easily accepting the rewriting process.
It naturally follows from the above discussion that driving a liquid crystal display device using a ferroelectric liquid crystal having a dielectric anisotropy (.DELTA..epsilon.) of less than 0 with the above driving method results in an improvement of the contrast and it is further expected that using C1 uniform further improves the contrast.
However, a ferroelectric liquid crystal actually manufactured with the above construction did not offer a high contrast as had been expected.
Far from offering a high contrast, it could not provide a favorable switching.
Thus, an examination was made on the cause for such result of failing in offering a high contrast and favorable switching with the result that it has been made clear that C1 uniform is not favorable for the above driving method and that a plurality of orientations are contaminated in one pixel. For example, FIG. 9 is a view illustrating an orientation of the liquid crystal material SCE-8 (manufactured by E Merck) used in a ferroelectric liquid crystal device having a cell width of 2 .mu.m subjected to the parallel rubbing as observed under a polarizing microscope. Driving the liquid crystal with the driving method published by P. W. H. Surguy et al. has clarified the presence of three portions, portion A exhibiting a favorable contrast, portion B exhibiting a contrast of 5 or less, portion C exhibiting no switching.
In view of the above, an object of the present invention is to provide a ferroelectric liquid crystal display device with a high contrast by optimizing a combination of the orientational sate and with unified orientation in whole pixels the driving method using a relationship between the applied voltage and the response rate as can be seen in a ferroelectric liquid crystal display device having a dielectric anisotropy of less than 0.